In order to avoid the disadvantages of conventional multi-jointed and metallic track chains for vehicles, an increasing amount of developmental effort has been directed to utilizing a continuous elastomeric or rubber belt entrained about a pair of longitudinally spaced wheels for propulsion and support. Positive drive between one wheel and the belt has also been heretofore preferred because driving the belt solely by friction requires a substantial area of angular contact or wrap about the driving wheel and a fairly high level of relatively constant tension of the belt. Unfortunately, prior systems of positive drive have unduly complicated the structure of the belt and have provided undesirable stress risers in the belt that greatly limit the service life thereof. Specifically, a plurality of holes rods, metallic elements or the like have usually been required in the belt for positively coupling with the protruding lugs on the drive wheel. On the other hand, a primarily friction-driven belt is highly desirable because there can be flew if any interruptions in the reinforcing plies used to transmit the desired tension and to provide the desired degrees of stiffness and structural integrity to the belt.
In high speed and heavy duty agricultural, earthmoving, and military applications, the demands upon such a belt are particularly great. While each belt must transmit a relatively large tension load sufficient to substantially eliminate slip between the driving wheel and the frictionally driven belt, it must simultaneously be circumferentially flexible to allow it to maintain compliance with uneven terrain. In the lateral direction, it should have a longitudinal stiffness sufficient for minimizing deflection or snaking thereof when obstacles are encountered and to better mate with the wheels. Lastly, in the transverse direction of its cross section, it should be resistant to bending.
U.S. Pat. No. 3,498,684 issued Mar. 3, 1970 to A. D. Hallaman illustrates one traction belt that recognized and attempted to overcome some of these problems. That disclosure taught a belt having spaced apart tension members in the form of longitudinally oriented and relatively inextensible textile cords or stranded steel wire, and transversely oriented reinforcing members of stranded steel wire on opposite sides of the tension members. However, the continuity of the belt was undesirably interrupted with penetrating apertures for receiving sprocket teeth and the belt was not constructed to resist oblique loads.
An earlier belt is illustrated in U.S. Pat. No. 2,476,828 issued July 19, 1949 to A. B. Skromme which desirably had no interruptions and three layers of reinforcement to provide the desired circumferential, longitudinal, and lateral stiffness characteristics. Specifically, it provided an outer first ply portion consisting of a generally circumferentially extending cable, an adjacent and intermediate second ply portion of parallel wire cables at a preselected inclined angle, and an inner third ply portion of parallel wire cables at a preselected inclined angle with the longitudinal plane of the belt and at an opposite and equal angle with the wire cables in the second ply portion. The juxtaposed second and third biased ply portions that reinforced the elastomeric material of the belt and served to resist deflection thereof by transversely directed forces at various angular attitudes was a substantial feature of U.S. Pat. No. 2,476,828.
Unfortunately, when belts having a circumferentially extending ply portion and oppositely biased ply portions of similar construction are operated under high tension loads and at high speeds in a bending mode about the wheel structures, the angled plies undesirably cause lateral shifting of the belts to one side. In other words, the belts have a tendency to move laterally or to drift in use away from a mid-circumferential plane and this causes undesirable friction against the belt guides and a buildup of heat that reduces belt service life.
Pneumatic tires have heretofore included biased breaker ply portions outwardly of one or more carcass ply portions and having various widths and various angular attitudes of the parallel cords or wires. Unfortunately, as the tire deforms in the "footprint" region, stresses arise as a result of the biased cords that have a transverse component. Such tires therefor exhibit unequal wear and want to steer to one side. U.S. Pat. No. 3,175,598 issued Mar. 30, 1965 to A. Cegnar discloses a tire with an outer breaker portion having a narrower width than an underlying breaker portion, and a modification wherein the inclination angle of the wires of the outer breaker portion should be less than 30.degree. and be less in magnitude than the inclination angle of the wires of the underlying breaker portion. U.S. Pat. No. 3,357,470 issued Dec. 12, 1967 to J. Massoubre teaches use of tire crown reinforcement having cables in at least two adjacent ply portions which are angularly related in accordance with a specific mathematical equation so that the stresses developed are such as to be directed perpendicular to the cords of the plies to eliminate shearing stresses. However, when the mathematical methods used to solve these pneumatic tire deformation problems are applied to an endless vehicle drive belt, the theoretical results are unsatisfactory. Particularly, it is believed that the drive belts would exhibit at least as much tendency to drift laterally as existing belts, and in many cases the drift would be worse. It is apparent that the approach for designing the ply portions of a pneumatic tire are simply not directly applicable to a vehicular drive belt. On the one hand, solely rotatable pneumatic tires have a conventional horseshoe-shaped cross section and must provide a resiliently soft ride by a relatively flexible arrangement of elastic cords, many of which are in anchored carcass plies. On the other hand, vehicular drive belts preferably have a rectangular cross section, and are subjected to relatively high tension loads and transverse side loads when maneuvering. Such drive belts also extend between two or more wheel structures, and are usually wider than the wheel structures to minimize compaction of the earth and to provide the desired drawbar pull characteristics.
Accordingly, what is needed is an economical and reliable endless elastomeric or rubber drive belt of simple construction that will employ internal reinforcement in the form of breaker layers and/or ply portions so constructed and arranged as to provide the desired circumferential, longitudinal, lateral and oblique strength characteristics while simultaneously minimizing any self tendency thereof to shift laterally in use during bending about two or more wheel structures. Preferably, the belt thus provided should have a minimum number of uninterrupted ply portions and a relatively uniform cross sectional shape about its circumference.
The present invention is directed to overcoming one or more of the problems as set forth above.